This invention relates to a process for jet-cooking and spray-drying water-dispersible or water-soluble polymers, especially high viscosity starches such as high amylose starches. It also relates to the unique pregelatinized high amylose starches produced thereby.
Pregelatinized starches (i.e., cold-water-dispersible starches) are typically prepared by thermal, chemical, or mechanical gelatinization. The term "gelatinized" or "cooked" starch refers to swollen starch granules which have lost their polarization crosses and which may or may not have lost their granular structure.
The thermal processes generally used to prepare such starches include batch cooking, autoclaving, and continuous cooking processes in a heat exchanger or jet-cooker. The thermal dispersion of a granular starch in water involves a complex mechanism. See the discussion at pp. 427-444 in Chapter 12, by Kruger & Murray of Rheology & Texture in Food Quality, Edited by T. M. DeMan, P. W. Voisey, V. F. Rasper, and D. W. Stanley (AVI Publishing, Westport, Conn. 1976), at pp. 449-520 in Chapter 21 of Starch: Chemistry & Technology, Vol. 2, edited by R. L. Whistler (Academic Press, New York, N.Y. 1967) and at pp. 165-171 in Chapter 4 by E. M. Osman of Food Theory and Applications, edited by P. C. Paul and H. H. Palmer (John Wiley & Sons, Inc., New York, N.Y. 1972). The process begins at the gelatinization temperature, as water is absorbed into the starch granules, and continues as the hydrated granules swell and disrupt into smaller granule fragments until the starch finally approaches a molecular dispersion. The viscosity of the cook changes significantly during this process, increasing as the granules hydrate and swell and decreasing as the granular fragments are reduced in size. An appropriate amount of shear aids in breaking down the swollen granular fragments to give a molecular dispersion without substantial molecular degradation.
Pregelatinized starches are typically prepared by spray-drying, drum-drying, or extrusion.
Drum-drying involves simultaneously cooking and drying a starch slurry or paste on heated, rotating drums. Cooking and drying are accomplished over a period of time as determined by the temperature and rotation rate of the drums. The dried sheets are scraped off the drum with a metal knife and then ground. This process can be conveniently carried out at high solids content (typically about 43% maximum). The disadvantage of drum-drying is that this method generally only partially disperses the starch (i.e., the starch is not completely gelatinized) and this can result in poorly dispersible powders having undesirable textures when redispersed.
Extrusion may also be used to simultaneously cook and dry starches (see U.S. Pat. No. 3,137,592 issued Jun. 16, 1964 to T. F. Protzman et al.). This method involves the physical working of a starch-water mixture at elevated temperature and pressure, causing the gelatinization of the starch, followed by expansion during flashing off the water after exiting from the extruder. The temperature and pressure are generated by mechanical shear between the rotating screw (auger) and cylindrical housing (barrel) of the extruder. Cooking is accomplished with both thermal and mechanical energy as the starch is forced through the system. This typically results in high viscosity during processing due to incomplete cooking and the final products are typically degraded due to molecular breakdown caused by excessive shear. Upon redispersion, the powders can give undesirable grainy textures, especially when low moisture starches are processed, due to incomplete dispersion during cooking. When starch is processed in the presence of additional water, a further drying step is required after the extrudate exits the extruder. This extended drying time further exaggerates the undesirable textures upon redispersion.
The following patents describe various processes for preparing pregelatinized starches.
U.S. Pat. No. 1,516,512 (issued Nov. 25, 1924 to R. W. G. Stutzke) describes a process for modifying starch in which starch slurries are forced through a heated pipe coil and then through a spraying orifice into a drying chamber. The slurries are processed with or without acid. The slurries are forced through the coil at excessively high pressure (e.g., 1000 lbs.) in order to insure against the possibility of vaporizing the liquid under treatment. Steam is maintained at 35-110 pounds of pressure. The temperature of the air introduced into the drying chamber is about 121.degree. C. (250.degree. F.), which is reduced to about 96.degree. C. (204.degree. F.) at the point of evaporation. The resulting starches are hydrolyzed and are about 15-75% soluble in cold water.
U.S. Pat. No. 1,901,109 (issued Mar. 14, 1933 to W. Maier) describes a spray-drying process in which starch slurries are atomized into a stream of heated air containing water vapor in such amount that vaporization of the water from the atomized particles occurs at a temperature above the gelatinization temperature of the starch and below the temperature at which further alteration (e.g., hydrolysis) occurs. The process can be carried out with or without a chemical gelatinization agent.
U.S. Pat. No. 3,630,775 (issued Dec. 28, 1971 to A. A. Winkler) describes a spray-drying process in which a starch slurry is maintained under pressure during heating and continued under pressure through the atomization step. The pressure is interdependent with viscosity, temperature, and apparatus. The pressure requirement is that necessary for atomization and is in excess of that necessary to prevent vaporization of water in a slurry of high solids at elevated temperatures. The heating time is that which is sufficient to allow substantially complete gelatinization and solubilization of the starch if previously ungelatinized. Typically, the slurries (10-40% solids) are preheated to 54.degree.-171.degree. C. (130.degree.-340.degree. F.), pumped under 2,000-6,800 psi of pressure through a continuous tubular heat exchanger, and heated to 182.degree.-304.degree. C. (360.degree.-580.degree. F.) (which result in starch temperatures of 163.degree.-232.degree. C.-325.degree.-450.degree. F.). Retention time of the starch in the cooker is 1.0-2.5 minutes. A conventional spray-dryer with a pressure type atomizing nozzle is used. The resulting starches are characterized as having less than 12% moisture, greater than 33 lb/ft.sup.3 bulk density, and greater than 50% cold-water solubility.
U.S. Pat. No. 4,280,851 (issued Jul. 28, 1981 to E. Pitchon et al.) describes a spray-drying process for preparing granular pregelatinized starches. In this process a mixture of the granular starch in an aqueous solvent is cooked or gelatinized in an atomized state. The starch which is to be cooked is injected through an atomization aperture in a nozzle assembly to form a relatively finely-divided spray. A heating medium is also injected through an aperture in the nozzle assembly into the spray of atomized material so as to heat the starch to the temperature necessary to gelatinize the starch. An enclosed chamber surrounds the atomization and heating medium injection apertures and defines a vent aperture positioned to enable the heated spray of starch to exit the chamber. The arrangement is such that the lapsed time between passage of the spray of starch through the chamber, i.e., from the atomization aperture and through the vent aperture, defines the gelatinization time of the starch. The resulting spray-dried, pregelatinized starch comprises uniformly gelatinized starch granules in the form of indented spheres, with a majority of the granules being whole and unbroken and swelling upon rehydration. Nozzles suitable for use in the preparation of these starches are also described in U.S. Pat. No. 4,610,760 (issued Sep. 9, 1986 to P. A. Kirkpatrick et al.).
U.S. Pat. No. 3,086,890 (issued Apr. 23, 1963 to A. Sarko et al.) describes a process for preparing a pregelatinized isolated amylose powder. It involves autoclaving a slurry of an isolated amylose having an intrinsic viscosity of 1.3-2.9 at 191.degree. C. (375.degree. F.) under 5-140 psig of pressure for 1-60 minutes at 0.1-25% solids, cooling the dispersion to 90.degree. C. (194.degree. F.), and drum-drying on a 110.degree.-200.degree. C. (230.degree.-392.degree. F.) surface. The retention time on the drum is 40-75 seconds using a nip gap of 0.001 inch or less. The resulting powders have amorphous X-ray diffraction patterns, intrinsic viscosities of 1.3-2.9, and form irreversible gels when redispersed.
Pregelatinized starches may be made by a two step spray-drying process which is in current industrial use. Modifications of this conventional process are described in U.S. Pat. No. 2,314,459 (issued Mar. 23, 1943 to A. A. Salzburg) and U.S. Pat. No. 3,332,785 (issued Jul. 25, 1967 to E. Kurchinke). In the typical process an aqueous starch slurry is cooked, usually by atmospheric vat cooking or by cooking in a heat exchanger or by steam injection jet-cooking, held at atmospheric pressure in a tank (often a cooking tank in batch processes or a receiver tank for pressurized cooking processes), and subsequently spray-dried. The post-cooking holding period allows the batchwise addition of additives, temperature regulation, and/or cooking at rates which do not match the spray-dryer capacity. On exiting the holding tanks the temperature of the feed to the spray-dryer may range from 38.degree.-93.degree. C. (100.degree.-200.degree. F.). Atomization is effected by a single fluid pressure nozzle, a centrifugal device, or a pneumatic nozzle. This process is usually limited to "thin-cooking starches", i.e., converted starches where the polymeric structure has been degraded by acid hydrolysis, enzymatic degradation, oxidation and/or high levels of mechanical shear. Converted starches can be used at higher solids because their pastes are lower in viscosity and can be atomized. The cooks of unmodified starches are difficult to atomize because of their high viscosity and therefore, if spray-dried, are processed at low solids. Another limiting factor of conventional processes is that, at temperatures achieved at atmospheric pressure, many polymers associate and/or retrograde causing an increase in viscosity. See U.S. Pat. No. 3,607,394 discussed below.
U.S. Pat. No. 3,607,394 (issued Sep. 21, 1971 to F. J. Germino et al.) is directed to a process for preparing a pregelatinized, cold water dispersible starch from a granular starch which contains at least 50% amylopectin (i.e., not more than 50% amylose). Suitable starches include cereal starches such as corn, wheat and barley, tuber starches such as potato and tapioca, and waxy starches such as waxy maize, waxy rice, and waxy sorghum. The high amylose starches, those which contain 60% or more amylose, as well as isolated amylose itself, are not suitable because their gelling characteristics are undesirable for the applications contemplated (i.e., where smooth pastes having a low initial viscosity and minimal setback). The process involves pasting at at least 149.degree. C. (300.degree. F.), with the upper limit being that at which substantial molecular degradation of the starch occurs, e.g., over about 232.degree. C. (450.degree. F.). The starch paste is then dried very rapidly in any suitable apparatus, e.g., a drum-dryer, spray-dryer, belt dryer, foam mat dryer or the like. The only requirement is that the apparatus be capable of drying the starch paste very rapidly to prevent retrogradation or aggregation prior to removal of the water. Also it is preferred that the paste be fed to the dryer very quickly because the longer it is held at a high temperature the greater is the likelihood of degradation. Structurally the products are characterized by complete granular fragmentation.
It is well known that high amylose starches are especially difficult to disperse and require higher temperatures and higher shear levels than low amylose starches such as corn, potato, wheat, rice, tapioca, and the like. Autoclaving or indirect heating, such as in a heat exchanger, are cooking processes that tend to produce dispersions that are complex colloidal mixtures, especially with the difficult to disperse high amylose starches. The mixtures consist of intact granules, residual granular fragments and dissolved polymer. Jet-cooking provides appropriate shear levels and more readily gives a dispersion approaching complete solubility at a molecular level (see U.S. Pat. Nos. 2,805,966 (issued Sep. 10, 1957 to O. R. Ethridge); 2,582,198 (issued Jan. 8, 1957 to O. R. Ethridge); 2,919,214 (issued Dec. 29, 1959 to O. R. Ethridge); 2,940,876 (issued Jun. 14, 1960 to N. E. Elsas); 3,133,836 (issued May 19, 1964 to U. L. Winfrey); and 3,234,046 issued Feb. 8, 1966) to G. R. Etchison). This more effective dispersion by jet-cooking provides a lower in-process viscosity, without degradation, than other cooking methods. This allows the use of lower cooking and conveying temperature and pressure which further assists in reducing degradation.
Therefore, there is a need for a spray-drying process which converts crystalline polymers to a substantially amorphous, i.e., "glassed" form, without substantial degradation by thoroughly cooking and drying water-dispersible or water-soluble natural polymers, such as unconverted starches and gums, or synthetic polymers such as polyvinyl alcohol at commercially acceptable solids concentration.
There is also a long felt need for a cooking and drying process that transforms cold-water-insoluble, partially insoluble, or slow to hydrate polymers (natural or synthetic) into new spray-dried powder forms which disperse in cold water and are substantially non-crystalline, non-retrograded and non-degraded by the process. The prior art teaches many methods that produce pre-dispersed polymers, but the resulting polymers do not possess the full range of desired properties. There is a need for a process which thoroughly cooks and spray-dries crystalline polymers, such as converted starches, at higher solids than is currently possible.
In particular, there is a need for the following:
i) pregelatinized, spray-dried, fully pre-dispersed high amylose starches (modified or unmodified) which disperses in water (i.e., high amylose starches which are substantially cold-water-soluble and completely hot-water-soluble) and whose redispersions give strong gels with improved textural properties;
ii) fully pre-dispersed, spray-dried forms of modified or unmodified natural gums (which are inherently poorly dispersible due to crystalline or associated regions), especially polygalactomannan gums whose backbones are more linear in nature and have a tendency to associate to form crystalline regions, such as locust bean gum, and whose spray-dried forms yield cold-water redispersions with the solution properties of the parent gum.
iii) fully pre-dispersed, spray-dried forms of synthetic polymers which are inherently poorly dispersible due to crystalline or associated regions, especially substantially fully hydrolyzed polyvinyl alcohols and whose spray-dried forms yield cold-water redispersions with the solution properties of the parent polymer.